Simulacrum states:
If you cast this spell again, any currently active duplicates you created with this spell are instantly destroyed.
Does this mean that if I take my simulacrum and permanently transform it using true polymorph into, say, a dragon, that if I cast simulacrum again, it would still instantly destroy that dragon I just produced?
tl;dr True polymorph changes a targets form and stat block. The property of being a simulacrum is not inherent in either of these, so true polymorph does not insulate nor prevent the simulacrum from being destroyed upon a subsequent casting.
It's flexibility with regard to form is already established as it can duplicate any beast or humaniod. Regardless of form, it's still a simulacrum.
You shape an illusory duplicate of one beast or humanoid
the new form can be any kind you choose whose challenge rating is equal to or less than the target's
The target gets the statistics of the new form.
The target's game statistics, including mental ability scores, are replaced by the statistics of the new form. It retains its alignment and personality.
Being a simulacrum is specified by a magical effect not the stat block of the form it assumes. So regardless of form or stat block, it's still a simulacrum subject to the effects of the spell that created it including being destroyed upon subsequent casting.
yes, your Simulacrum is even when true polymorphed just affected by a spell that can be negated by other means, it is still your simulacrum and would be destroyed. The simulacrums [I´m a Simulacrum] property is not a game statistic that can be changed.
Objects or creatures Truly Polymorphed are no longer 'currently active duplicates' and would not targeted for 'instant destruction'.
The 5e spell Simulacrum states:
If you cast this spell again, any currently active duplicates you created with this spell are instantly destroyed. (emphasis added / page 276 Player's Handbook)
It is believed this line was added so casters could not make infinite copies of themselves. This was a hot topic & all the rage a few years back. When the PhB was reprinted, this line was added so as to give this high-magic fantasy role-playing game far more realism, verisimilitude and the respect it deserves (yet still keeping the beast-master ranger exactly as it was, of course).
So now, when Simulacrum is re-cast (specifically) on the exact same target-humanoid &/or target-beast, this magic will also (as a bonus feature?) somehow search all known universes for any possible 'current active duplicates'. Should it find them, it instantly wipes them out (or at least turns them into rapidly melting snow sculptures). Of course, anything subject to a True Polymorph could not possibly be a recognizable duplicate of anything anymore. In fact, StackExchange has made it perfectly clear that one cannot even True Polymorph back into oneself.
